This invention relates to novel triphenolic compounds that have utility as branching agents and thermoplastic, randomly branched polycarbonates having excellent resistance to thermal oxidation and excellent blow molding properties and to a process for their preparation.
Polycarbonates are well known, commercially available materials which have achieved wide acceptance in the plastics industry. Such polymers are prepared by reacting a carbonate precursor such as phosgene, for example, with a dihydric phenol such as 2,2-bis (4-hydroxyphenyl) propane, herein referred to as "bisphenol-A," to provide a linear polymer consisting of dihydric phenol derived units bonded to one another through carbonate linkages. Generally speaking, such polymers offer a high resistance to mineral acids, have a high tensile strength and a dimensional stability and impact strength far surpassing that of any other thermoplastic material.
These aromatic polycarbonates differ from most thermoplastic polymers in their melt rheology behavior, in that they, in contrast to most thermoplastic polymers, exhibit Newtonian flow at normal processing temperatures and shear rates below 300 reciprocal seconds.
Most thermoplastic polymers exhibit non-Newtonian flow characteristics over essentially all melt processing conditions. Newtonian flow is the type of flow occurring in a liquid system when the rate of shear is directly proportional to the shearing force, i.e. there is a constant value of viscosity. Non-Newtonian flow occurs when the viscosity varies with shear rate.
Two other characteristics of molten thermoplastic polymers are significant for molding; these are melt elasticity and melt strength. Melt elasticity is the recovery of the elastic energy stored within the melt because of distortion or orientation of the molecules by shearing stresses. Melt strength may be described as the tenacity of a molten strand and also as the ability of the melt to support a stress. Both melt elasticity and melt strength are important properties in extrusion blow molding, particularly in fabrication by extrusion blow molding.
Non-Newtonian flow characteristics tend to impart melt elasticity and melt strength to such polymers, allowing the use thereof in blow molding fabrication. In the usual blow molding operation, a hollow tube of molten thermoplastic is extruded vertically downward at a temperature of about 200.degree.-400.degree. C. A mold then surrounds the tube and gas is introduced into the tube to force it to conform to the shape of the mold. The length of the tube and the quantity of material comprising the tube are limiting factors in determining the ultimate size and wall thickness of the molded part.
The fluidity of the melt obtained from bisphenol-A polycarbonate, the relatively low melt strength and also the paucity of extrudate swelling, serve to limit blow molding applications to relatively small, thin walled parts. Temperatures must also be extremely carefully controlled to prevent the desired length of extruded tube from falling away before the mold can close around it for blowing. Consequently, it will be appreciated that the Newtonian behavior of polycarbonate resin melts has served to restrict severely their use in the production of large hollow bodies by conventional extrusion blow-molding operations as well as in the production of various shapes by profile extrusion methods. Thus, it is desirable to form polycarbonates which provide melts with increased stability during molding at elevated temperature.
Thermoplastic, randomly branched polycarbonates, exhibit unique properties of non-Newtonian flow, melt elasticity and melt strength which provide such stability and permit them to be used in molding operations to obtain such articles as bottles which were not heretofore easily or readily produced with linear polycarbonates. The thermoplastic, randomly branched polycarbonates can be prepared by reacting a polyfunctional compound containing three or more functional groups with a dihydric phenol and a carbonate precursor.
Several prior art disclosures such as exemplified by U.S. Pat. Nos. 2,950,266 and 3,030,335 concerning the addition of a trifunctional additive to polycarbonate forming reactions between dihydric phenols and carbonyl halides teach that if a cross-linked product does not occur as a direct result of the initial polymerization reaction, the final reaction product of the dihydric phenol, the trifunctional compound, and the carbonyl halide would be a heat curable product.
Other prior art attempts have been made to incorporate a polyfunctional compound into polycarbonates of dihydric phenols as exemplified by U.S. Pat. Nos. 3,094,508 and 3,544,514. These have been limited to the preparation of high molecular weight polymers under very limited process conditions. Their preparation requires carefully controlled process conditions which are both cumbersome and expensive. Additional processes are disclosed in U.S. Pat. No. 4,001,184. Other attempts have been made to provide polycarbonate resins which exhibit non-Newtonian melt characteristics as, for example, disclosed in U.S. Pat. No. 3,166,606. However, the polycarbonates there disclosed consist of a physical blend of two or more polycarbonate resins having differing values of reduced viscosity. Moreover, the individual polycarbonate components disclosed by the patentees in the production of such polycarbonate blends are produced entirely from difunctional reactants. Tetraphenolic compounds are obtained from monofunctional phenols and dione precursors in U.S. Pat. No. 4,277,600.